I. Technical Field
The present invention relates to a forming method for pipe bodies having necked portions at pipe ends, which are used for converter cases or mufflers of automobiles.
II. Description of Related Art
For cases of exhaust gas cleaning catalyst converters or cases of silencer mufflers mounted to exhaust systems of automobiles, pipes of large diameter are used due to a need for larger capacity. Ends of the case member have tapered portions for connecting to adjacent members and, as needed, straight pipe portions of small diameter that are continuous with the tapered portions.
FIG. 1 shows an example of a case 1 with a portion (1c) of a pipe of a large diameter, which is a pipe stock portion, tapered portions (1b) provided for connecting to the adjacent members, and straight pipe portions (1a) of small diameter continuous with the tapered portions. As a forming method therefore, a spinning method is used in many cases.
The spinning method is a method by which working rollers as forming tools are brought into contact with a surface of the working object pipe. While being relatively revolved around the working object pipe, the working rollers are driven in both the radius direction and the axial direction of the working object pipe, thereby forming the tapered portions, which are gradually reduced in diameter toward the ends of the working object pipe, and, as needed, the straight pipe portions of small diameter continuous therewith.
For example, JP 11-132038A discloses that the spinning is performed by pressing the working rollers against the outer surface of the working object pipe, thereby manufacturing a converter case.
Behind the frequent adoption of the spinning method as a method for forming members of the above-mentioned shape, there are the following merits. That is, compared to a method of weld bonding plate materials after press forming, 1) material yield is high, 2) production efficiency is high, 3) member strength is high due to seamless forming, 4) and welding is not required, so reliability of the member is not degraded due to a welded portion, etc.
However, in the spinning method, the working rollers are repeatedly reciprocated in the axial direction of the working object pipe, consequently, the material tends to move toward the pipe end due to plastic deformation, and the thickness of the tapered portion tends to be locally decreased, compared to the ordinary press forming method. Further, fractures originated from the pipe working ends, where the diameter is reduced, tend to occur, at the base portion, or, in case of electric resistance welded pipes, at the welded portion. Furthermore, wrinkles, the nonuniform deformation at the changing the circumferential length of the pipe, tend to be generated during the spinning process, and thus the predetermined working shapes cannot be obtained.
For example, JP 2003-342694 and JP 2004-243354 each suggests a technology, that will suppress the occurrence of crackings originated from the working end of the base portion or the welded portion by adjusting material compositions of the steel, or by regulating the cross-sectional shape of the welded portion, in a case of using ferritic stainless steel pipe.